This invention relates to injection molding and more particularly to an improved seal positioned between the nozzle and the gate.
It is well known that the temperature of the melt in the gate area is critical to satisfactory operation of the system, particularly with materials having high temperature characteristics. Therefore, it is desirable to maintain the melt temperature as it flows into the gate area, but at the same time excessive heat transfer to the cooled cavity plate must be avoided.
In the past, this has been provided by having the melt flow around a central heated torpedo member leading to the gate. However, such an internal heating system has the disadvantage that the melt passage is restricted by the torpedo along a considerable portion of its length resulting in unacceptable pressure drop for most materials. In addition, there is a very high temperature drop across the relatively narrow melt passage from the heated torpedo to the cooler surrounding material. This requires excessively high melt temperatures at the heated torpedo which may result in melt breakdown.
More recently, systems have been provided which are externally heated with a central conductive pin which is not heated extending in the melt to the gate area. While this does avoid the temperature gradient problem, it does not avoid the pressure drop problem. Even more recently, attempts have been made to overcome these problems by providing an externally heated nozzle with an integral central hot tip extending to the gate area. While this, at least partially, overcomes the above mentioned two problems, it introduces the problem of excessive heat loss from the heated nozzle to the surrounding cooled cavity plate. While a space between the nozzle and the cavity plate may be allowed to fill with melt to act as an insulator, this causes problems on colour changes and also may result in deterioration of the trapped melt. A nozzle housing may be provided between the heated nozzle and the cooled cavity plate, but this has the disadvantage of being a considerable added expense.